O'Neill believes a match at Pride Park could have been significant in Petrov's development.
He said: "The turning point to his career might well have been the back end of the season before last when he scored a really good goal against Derby.
"Basically, he was coming in to play a more central role in midfield.
"He had been previously playing on the right hand side when maybe when we only had one wide player at the time. He was coming in as a third midfield player having to break wide and not being in a central position.
"I think his form now is mainly down to his own confidence which he had to regain and which he has done splendidly."